Preparation of oxalic acid

ABSTRACT

OXALIC ACID IS MADE BY OXIDIZING PROPYLENE WITH AT LEAST 50% NITRIC ACID AT 40 TO 80*C.

March 21, 1972 J. BQICHARD HAL 3,651,135 PREPARATION OF OXALIC ACIDOriginal Filed April 4., 1966 A tlorneys United States Patent 1 ,311Int. Cl. C07c 55/06 U.S. Cl. 260-533 R Claims ABSTRACT OF THE DISCLOSUREOxalic acid is made by oxidizing propylene with at least 50% nitric acidat 40 to 80 C.

The present invention relates to the preparation of oxalic acid byoxidation of propylene.

This application is a continuation of Ser. No. 539,775, now abandoned.

The process most commonly employed in industry for the preparation ofoxalic acid is the alkali metal formate process, which consists indecomposing alkali metal formates, such as sodium formate, to alkalimetal oxalates by heatin converting the alkali metal oxalates intoinsolu ble calcium oxalate and then liberating the oxalic acid withsulphuric acid. Despite the good yields of oxalic acid which it gives,this process has certain disadvantages from the industrial viewpoint: itis carried out in a number of phases, and there is a high consumption ofsulphuric acid converted into calcium sulphate, which is a by-productthat is difiicult to recover.

It has also been proposed to prepare oxalic acid by oxidation ofunsaturated hydrocarbons such as propylene. Thus, in German Pat.742,053, a process is described for the oxidation of propylene withnitric acid at a temperature between 50 and 70 C., in the presence orabsence of oxygen, under normal or higher pressure.

The best yields (80% based on the propylene used up, and 76% based onthe propylene employed) have been obtained by using as oxidising agent amixture of sulphuric and nitric acids in the presence of oxygen, atnormal pressure. The presence of sulphuric acid involves technologicalproblems (e.g. recovery and concentration of the oxidation motherliquors, and considerable corrosion of the apparatus). In the absence ofsulphuric acid, the yields based on the propylene employed do not exceed51.5% using oxygen at normal pressure, or 64% under pressure, and do notexceed 44% in the absence of oxygen. It is also known (see U.S. Pat. No.3,081,345) that the oxidation of propylene by this process is not rapid,and that that portion of the propylene which is not combined in thereaction medium forms with the nitrous vapours (more particularly N0 agas mixture which gives rise to a danger of explosion.

In order to obviate the disadvantages of this process, it has beenproposed (in the aforesaid U.S. Pat. No. 3,081,345) to carry out theoxidation of the propylene in two stages: (1) oxidation of the propylenein liquid N0 between 30 and +21 0., leading to intermediate products;followed by (2) oxidation between 60 and 120 C. of the products of thefirst stage by an oxidation medium which is preferably a mixture ofsulphuric and nitric acids, but which may also be HNO or N0 (with theuse of pressure).

This latter process has the disadvantage that it takes place in twostages involving fairly different operating conditions, and above allthat it is dependent upon the Patented Mar. 21, 1972 useldof sulphuricacid for obtaining good oxalic acid yie s.

It has no been found that it is possible to obtain oxalic acid in goodyields in the absence of sulphuric acid and oxygen, by the oxidation ofpropylene with nitric acid, by passing gaseous propylene into a liquidmedium containing at least 50% by weight of nitric acid and at a.temperature of 40 to 80 C.

In the previously described processes, the nitric acid concentrationdecreases in the course of the reaction, on the one hand because of theincrease in the weight of the reaction mass and on the other handbecause of the consumption of nitric acid and its entrainment in thenitrous vapours. Moreover, it is found that, as the nitric acidconcentration decreases, the quantity of propylene entering into thereaction also decreases, which results in a considerable lowering of theyield and in the formation of a gaseous mixture of propylene and nitrousvapours.

On the other hand, maintaining the nitric acid concentration at 50% orabove makes possible a total conversion of the propylene and theproduction of excellent yields of oxalic acid, in the absence ofsulphuric acid. Moreover, it has been found that, in the process of theinvention, the quantity of acid which disappears in the form ofirrecoverable nitrogen is low. It is known that, when organic compoundsare oxidised with nitric acid, the latter is reduced in some cases asfar as the irrecoverable nitrogen and nitrous oxide (N 0), in additionto the recoverable nitrous vapours formed of NO and N0 variably combinedin the form of N 0 which may be oxidized to regenerate nitric acid. Theterm consumption of nitric acid means the quantity of nitric acidconverted into irrecoverable nitrogen or N 0; it is expressed in mol. ofHNO per mol. of oxalic acid formed.

Although the new process may be carried out with highly concentratednitric acid, it is possible to obtain good yields even with nitric acidof concentrations ranging from 60% to by weight, and the use of suchconcentrations is preferred.

Various methods may be adopted to ensure that the nitric acidconcentration is maintained above 50%. For example, the experimentallydetermined necessary quantity of fresh nitric acid may be introducedthroughout the duration of the reaction. In this case, in order not todilute the reaction medium, an acid of higher concentration than thestarting acid is preferably employed. It is also possible to combine theaddition of fresh nitric acid with a recycling through the reactor ofnitric acid obtained by oxidation and treatment with water of thenitrous vapours produced by the reaction.

The temperature of the reaction depends upon the nitric acidconcentration in the reactor. Generally speaking, the operation may becarried out at temperatures ranging from 40 to C., and preferably from50 to 70 C.

In practice, the process of the invention may be carried out as follows:

The reactor is charged with a predetermined amount of nitric acid, andpropylene is gradually introduced in highly divided form (i.e. as a veryfine stream of bubbles) and at a rate which, for a predeterminedconstant concentration of nitric acid in the reactor, depends upon thespeed of the reaction (and therefore upon the temperature). The maximumrate of supply is that at which propylene begins to be detectable in theefiluent gases.

Simultaneously, fresh nitric acid, and where appropriate regeneratednitric acid, are added to maintain the nitric acid concentration of themedium at a fixed value of 5 0% or above.

When the addition of the propylene is complete, the reaction medium issubjected to a finishing to complete the oxidation of the propylenetaken up by the reaction medium. During this phase of the process, it isunnecessary to maintain a constant nitric acid concentration in themedium. Generally speaking, the temperature of the finishing phase maybe the same as that adopted for the addition of the propylene. Theduration of the finishing phase depends upon the temperature. Thisfinishing makes possible a considerable improvement in the oxalic acidyield without increasing the consumption of nitric acid as irrecoverablenitrogen in relation to the amount of propylene introduced. At the endof the finishing phase, the reaction mass is cooled and dihydratedoxalic acid precipitates as white crystals.

Depending upon the operating conditions, up to 90% of the acid in thereaction medium may be isolated by simple filtration. The isolatedoxalic acid is washed with water and dried by heating to constantweight. It is found, and this is another advantage of the new process,that the oxalic acid obtained in this way has a purity of 100%.

The reactants and the operating conditions employed in the processaccording to the invention lend themselves particularly well tocontinuous industrial working.

The following examples illustrate the invention.

EXAMPLE 1 The apparatus employed is illustrated in the accompanyingdrawing. It is composed of the following elements: a cylindrical glassreactor 1 having a height of 520 mm., a diameter of 40 mm. and a usefulvolume of 700 cc.; a conical chamber 2 connected to the lower part ofthe reactor and connected thereto by a plate 3 of fritted glass No. 3(porosity 15-40;); a pipe 4 for the supply of propylene extending fromthe apex of the conical chamber, rising laterally along the outer wallof the reactor and connected to a propylene cylinder; a reactor headconsisting of a ground glass stopper 5 provided With a thermometer 6, apipe 7, for the admission of oxygen or inert gas, which terminates abovethe level of the reaction mass, and a Y-shaped pipe 8 connected on theone hand to a dropping funnel 9 and on the other hand to a column 10packed with Raschig rings of a diameter of 28 mm. and a height of 80mm., on which is mounted a coiled condenser 11 (height 150 mm.) suppliedwith ice-cold water (250 litres per hour) and the atmosphere of whichmay be placed in communication with an oxygen source 12; a column 13(height 400 mm., diameter 58 mm.) packed with Raschig rings, below whichis mounted a round-bottomed receiver flask 17 which is provided, on theone hand, with a lower lateral pipe connected to the upper part of thecondenser 11 and adapted to be connected to the oxygen source 12, and,on the other hand, with an upper lateral pipe; a circulating pump 14between the receiver flask 17 and the top of the column 13; a column(height 200 mm., diameter 31 mm.), also packed with 'Raschig rings, onwhich is mounted a distilled-water reservoir comprising a flowregulating valve in the connecting tube; a spherical receiver flask 18disposed below the column 15 and connected to the upper lateral pipeextending from the column 13; a bath 19 with thermostat intended forheating the reactor.

Before commencement of an operation, distilled water is introduced intothe reservoir 16 and into the flask 17, the bath is adjusted to atemperature of 50 C. and, while a light current of propylene is passedinto the reactor to avoid any admission of liquid into the chamber 2,539.8 g. of 72.2% nitric acid (i.e. 6.20 mol. of HNO are introduced intothe reactor through the dropping funnel 9.

When the temperature in the reactor reaches 50 C., the rate of supply ofpropylene is adjusted to 1.4 litre per hour (0 C./760 mm. Hg).

Oxygen is simultaneously admitted to the outlet of the coiled condenserat a rate of 5.3 litres per hour to ensure oxidation of the nitrousvapours emanating from the reactor; part of the vapours is condensed bythe condenser 11, the coil of which is cooled by a current of ice-coldwater, and returns to the reactor; the remainder of the vapours entersthe absorption system consisting of the column 13 sprinkled with liquidtaken from 17 by the recycling pump 14 and on the other hand through thecolumn 15 which is sprinkled dropwise by water coming from 16.

After reaction at this rate for 30 minutes, 93% nitric acid is graduallyintroduced into the reactor through the funnel 9. 17.8 g. of propylene(0.424 mol.) are passed through in seven hours, while the total quantityof additional 93% nitric acid amounts to 123.4 g. No trace of propyleneis found in the gases leaving the apparatus.

The reaction mass is then cooled to 25 C., and there is passed throughthe reactor, above the level of the liquid, for about 20 minutes, acurrent of oxygen at a rate of 10 litres per hour to ensure thedegassing of the apparatus and the oxidation of the nitrous vapours. Thereaction mixture is then weighed and a test is made to determine thenitric acid concentration of the reaction mass, and another to determinethe oxalic acid formed.

The weight of the reaction mass is 616.1 g. The mass is introduced intoa three-necked, round-bottomed onelitre flask equipped with a stirringsystem and a thermometer, and this flask is placed in the precedingapparatus in place of the reactor. The temperature of the products israised to 50 C. and a current of oxygen is simultaneously admitted (rateof flow 2.3 litres per hour) to the outlet of the coiled condenser. Atthe end of 4 hours, a test is made to determine the oxalic acid formed,and the finishing is then continued for 19 hours at 50 C.

The contents of the flask are cooled to 25 C. and degassed as before.The weight of the mixture is 578.8 g., i.e. a loss of 84.4 g. during thefinishing phase. Two tests are made, one to determine the oxalic acidand the other to determine the nitric acid concentration in the reactionmass.

Determination of the oxalic acid is effected in the conventional mannerafter it has been isolated by precipitation as calcium oxalate.

The nitric acid balance is established by nitrometric determination bymeans of the Lunge volumenometer, on the one hand on the reactionmixture after finishing, and on the other hand on the nitric acidliquors resulting from the oxidation of the nitrous vapours in thecourse of the reaction and in the course of the finishing.

The balance of the reaction is established as follows:

Oxalic acid yield calculated on the propylene employed: beforefinishing, 59%; after finishing, 72.2%. Nitric acid concentration in thereactor: before finishing, 64%; after finishing, 61%.

Recoverable nitrogen in the reactor, expressed in mol. of HINO afterfinishing, 5.62.

Recoverable nitrogen in the total of the acid liquors, expressed in mol.of HNO 1.86. I

The consumption of nitric acid in the form of irrecoverable nitrogenexpressed in mol. of HNQ per mol. of oxalic acid is thus 1.75 mol./mol.

The reaction medium is left for 16 hours at 5-10" C. Dihydrated oxalicacid precipitates as white crystals. The precipitate is filtered off,washed with cc. of ice-cold water on a filter and then dried at 40 C. toconstant weight, 25.8 g. of 100%. pure product are recovered.

EXAMPLE 2 In the apparatus of Example 1, a test was carried out,

the conditions and results of which were as set out in the followingtable:

Nitric acid introduced: 586 g. at 67% (6.24 mol. of

HNO

Temperature: 50 C.

100% propylene: rate of flow: 1.5 l./h. on average (at 0 C./ 760 mm. Hg)duration: 7 hours; weight: 19.2 g. (0.457 mol.)

Oxygen: rate of flow: 5.3 l./h.

Nitric acid added: 150 g. at 93.5%

Weight of the reaction mass before finishing: 687 g.

Finishing: 23 hours at 50 C.

Weight of the reaction mass after finishing: 645 g.

Weight of anhydrous oxalic acid before finishing: 23 g.;

56% yield calculated on the propylene employed Weight of anhydrousoxalic acid after finishing for 23 hours: 28.3 g.; 69% yield calculatedon the propylene employed Nitric acid concentration in the reactor,before finishing:

After finishing: 57.5%

Recoverable nitrogen in the reactor expressed in mol. of

HNO after finishing: 5.88

Recoverable nitrogen in the acid liquors in mol. of HNO Nitric acidconsumption in the form of irrecoverable nitrogen, in mol. of HNO permol. of oxalic acid: 1.59

Weight of dihydrated oxalic acid (100%): 26.3 g.

EXAMPLE 3 In the apparatus described in Example 1, the reactor isreplaced by a reactor having the following characteristics:

Height: 450 mm. Diameter: 30 mm. Useful volume: 300 cc.

and which has a double jacket for the circulation of water atthermostatically controlled temperature.

In to the reactor are charged 392 g. of 75% nitric acid, the temperatureof which is raised to 50 C. by circulation of water through the doublejacket. Propylene containing 8% of propane is then passed into thereactor at a rate of fiow of 4.94 litres per hour, i.e. 4.54 litres perhour 100% propylene. Gas extractions are effected every 90 minutes atthe outlet of the column for the recovery of the nitrous vapours, andchromatographically analysed. Extractions are simultaneously made fromthe reactor to determine the nitric acid concentration.

The following results were obtained:

Percent of propylene In the Absorbed HNOs. percent effluent in the inthe Extraction at the time taiter the commencement of the reaction gasesreactor reactio mass 1 hour 30 minutes- 0 100 64. 9 3 hours Traces 10051. 7 4 hours 30 minutes 8. 37 84 39. 7 6 hours 15. 5 32.3 8 hours 35.17 35 28. 3

4. Process according to claim 1, in which the propylene is introducedinto the reaction medium as a very fine stream of bubbles at a rate suchthat no propylene is detectable in the gas leaving the reaction medium.

5. Process according to claim 1 in Which after introduction of propyleneinto the nitric acid has ceased, the temperature of the latter ismaintained at 40 to C. to complete the oxidation of the propylene tooxalic acid.

References Cited FOREIGN PATENTS 742,053 10/1943 Germany 260-533LORRAINE A. WEINBERGER, Primary Examiner R. D. KELLY, Assistant Examiner

